Thomas L. Bean, in collaboration with Timothy W. Butcher and Timothy Lawrence
Wood dust is created when machines are used to cut or shape wood materials. Industries that have a high risk of wood-dust exposure include sawmills, dimension mills, furniture industries, cabinet makers, and carpenters. Negative health effects have been associated with professions that shape, cut, or work wood. Companies need to be aware of the health effects of wood dust, as well as NIOSH and ACGIH exposure level recommendations and applicable OSHA standards, and how they may affect their production facility.
The terms hardwood and softwood have no reference to the actual hardness of the wood. Hardwoods are from deciduous broad-leafed trees, and softwoods are from conifers. A significant difference does exist in the effect of the dust created during their handling. Hardwoods such as oak, mahogany, beech, walnut, birch, elm, and ash have been reported to cause nasal cancer in wood-workers. This is particularly true when exposures are high.
The American Conference of Governmental Industrial Hygienists (ACGIH) recognizes wood dust as a "confirmed" human carcinogen and recommends a limit of 1 milligram per cubic meter (mg/m3 ) for hardwoods and 5 mg/m3 for softwoods. At this time, OSHA regulates wood dust as a nuisance dust; however, OSHA strongly encourages employers to keep exposures to a minimum and to adopt the ACGIH levels. The maximum permissible exposure for nuisance dust is 15 mg/m3, total dust (5 mg/m3, respirable fraction).
Exposure to wood dust may cause external and internal health problems. Adverse health effects associated with wood dust exposure include dermatitis, allergic respiratory effects, mucosal and non-allergic respiratory effects, and cancer.
Allergic respiratory problems can be caused by wood dust. The chemicals in wood that are associated with allergic reactions are generally found in the inner parts or heartwood of the tree. A hypersensitivity reaction leading to asthma has been reported as a result of exposure to commonly used woods, including Western Red Cedar, Cedar of Lebanon, Oak, Mahogany, and Redwood. The asthmatic reaction is believed to be species-specific.
Dermatitis is also a common health hazard associated with exposure to wood dust. Wood, usually as sawdust or splinters, may affect the skin or mucous membranes by mechanical action or by chemical irritation and sensitization. Irritant reactions appear to be more common among lumber workers. The main population of workers who suffer from dermatitis-related problems are those who work in secondary wood product manufacturing facilities, although cases have been documented in sawmill workers.
Cancers have been associated with wood dust exposure. The National Institute for Occupational Safety and Health (NIOSH) considers both hardwood and softwood dust to be potentially carcinogenic to humans. The three types of cancers associated with wood dust exposure are nasal and sinus cavity cancer, lung and other cancers, and Hodgkin's disease. The wood and cancer relationship was studied by Milham (1974), who conducted a mortality study involving the AFL-CIO United Brotherhood of Carpenters and Joiners of America. This study supports the hypothesis that wood contains carcinogenic agents. The cancer mortality patterns found were:
Hodgkin's disease has also been associated with wood dust. One study (Milham & Hesser, 1967), which examined 1,549 white males terminally ill with this disease, showed an association between Hodgkin's disease and wood dust exposure. Another study (Spiers, 1969) concluded that men working in wood industries in the eastern United States were at special risk for the disease, due principally to the carcinogenicity of pollen grains from eastern pine species.
Western Red Cedar occupies a particular place in hazard awareness because it contains the irritant chemical plicatic acid. Plicatic acid is most concentrated in western red cedar, but it is also found in significant quantities in eastern white cedar and japanese cedar. Plicatic acid is believed to be the causative agent in western red cedar dust-induced asthma and affects between 4 and 13.5% of exposed populations (Chan-Yeung, 1994).
When the Occupational Safety and Health Act was passed in 1970, PELs for about 400 different substances were incorporated into the Act and became law. In 1985 OSHA was petitioned by the United Brotherhood of Carpenters and Joiners of America of the AFL-CIO to create a standard to protect workers from wood-dust levels deemed unsafe by the union. The union's proposed standard for wood dust set exposure limits at 1 mg/m3 for hardwoods and 5 mg/m3 for softwoods. The Forest Industry contended that the union's request would cost wood products manufacturers up to $1.5 billion per year and would ultimately reduce the number of manufacturers in the wood industry. After reviewing the health evidence presented, OSHA's finding was that a PEL of 1 mg/m3 for hardwoods was not warranted.
OSHA does not have a specific PEL for wood dust. However, the National Institute for Occupational Safety and Health (NIOSH) has established a recommended exposure limit (REL) for wood dust, all soft and hardwoods except western red cedar, of 1 mg/m3 as a TWA for up to a 10-hour workday and a 40-hour workweek [NIOSH 1992].
The American Conference of Governmental Industrial Hygienists (ACGIH) has assigned wood dust, all soft and hardwoods except western red cedar, a threshold limit value (TLV) of 1 mg/m3 for certain hardwoods, such as beech and oak, and 5 mg/m3 for soft wood, as TWAs for a normal 8-hour workday and a 40-hour workweek and a short-term exposure limit (STEL) of 10 mg/m3 for soft wood, for periods not to exceed 15 minutes. Exposures at the STEL concentration should not be repeated more than four times a day and should be separated by intervals of at least 60 minutes [ACGIH 1994, p. 36].
The ACGIH has assigned western red cedar dust a TLV of 0.5 mg/m3 because of its suspected involvement as an asthmatic trigger and sensitizer [ACGIH 2004].
Compliance with the PELs is required as last amended, August 4, 1997.
Compliance with the PELs was initially achievable by means of using "any reasonable combination of engineering, administrative, and respirator control methods" (Department of Labor, 1989). However, after December 31, 1993, compliance required the implementation of engineering controls.
Engineering control methods should be implemented before considering any other type of control. As of December 31, 1993, administrative controls and personal protective equipment may no longer be used as a means to comply with PELs. Central exhaust ventilation is the primary engineering control method.
Central exhaust systems are usually designed for specific operations in which the wood dust is captured at the machines and conveyed through an overhead piping system to a collector. For indoor applications, these systems can be designed with a heat exchanger that returns the heat or conditioned air to the room.
Dust collectors for individual machines are usually more expensive and require more maintenance. Temporarily closing ducts that service equipment not in operation increases air flow to the rest of the hoods or vacuum devices. Adding a stronger fan to the system may be another solution.
A ventilation engineer can often recommend a number of modifications to improve the performance of a specific ventilation system. Small changes or modifications to an existing ventilation system will generally be less costly than replacing the system.
Process or operator enclosure is another engineering control method. Operator enclosure can be used where process equipment is operated from a remote or semi-remote location. An example of this is a sawmill where the headrig and resaw operators work effectively from an enclosure or booth. This type of engineering control is not feasible in many situations.
Process enclosures are commonly used to reduce the noise levels of a piece of machinery. A wooden box can be constructed around the piece of equipment with insulation installed on the inside to dampen the noise level. This type of enclosure can also be used to reduce wood dust levels. Caution must be taken when using this type of modification because of the potential for equipment overheating due to inadequate ventilation.
Administrative controls include good housekeeping procedures. The use of compressed air for cleaning dust off equipment and other surfaces contributes significantly to employee exposure to wood dust. The alternatives to the use of compressed air include sweeping or vacuuming. For controlling wood dust exposure, vacuuming is preferred. Due to cost, it may be difficult to justify vacuuming as a substitute for compressed air; but, by experimenting with different vacuum attachments, an industrial vacuum can be made very efficient. Vacuums can also be used as an alternative to sweeping and using compressed air for removing dust from employees' clothing.
Proper maintenance is another control method. The proper combination of machine, tool, and work piece and proper machine operation can prevent unnecessary dust emissions. Local exhaust ventilation and air cleaning systems should be designed and maintained to prevent the accumulation of wood dust and the recirculating of wood dust into the work place. The ventilation system should be inspected periodically for effective performance.
Another cause of poor dust collection is the open sides of some machines or the openings at the cutterheads, such as those found on molders. In this case, air is drawn in from the sides rather than over the cutterhead where dust can be effectively collected. Also, when the cutting tools become dull, the radius of the cutting edge increases, causing the cutting tool to rub and crush the wood fibers rather than severing them cleanly. Smaller particles and more respirable dust result from poor tool geometry.
Good maintenance should be a priority and may contribute to improved productivity as well as provide reductions in dust levels. Other good practices include maintaining clean workspaces, wearing protective clothing, and avoiding skin contact to help prevent allergic reactions. Rotating jobs, classified as an administrative control, can reduce the amount of exposure by not allowing employees to work a full eight-hour shift in a high-dust level area. However, the rotation of workers is not considered a favorable safety and health procedure by OSHA.
When effective engineering controls are not feasible or while they are being instituted, appropriate respirators may be used. OSHA mandates that a person may not be assigned the use of a respirator unless it has been determined that they are physically able to perform the work and use the equipment. A local physician will determine what health and physical conditions are pertinent. The respirator user's medical status must be reviewed periodically (for instance, annually). Employers who assign respirators to employees must follow procedures listed by OSHA in 29 CFR part 1910.134 for respirator use.
There are a number of ways to check the work place for excessive airborne wood dust. A visual check may immediately identify a problem. Look for dust collecting on equipment, clothes, face, and hair and around the breathing zone of workers. This will help determine where the dust is created and how to mitigate the problem. Sweeping the floors may also be causing unnecessary airborne wood dust.
A clean work area during equipment operation is a good indication that wood dust levels are below OSHA PELs. However, the only way to be certain is to monitor the air for wood dust. Air sampling records documenting compliance may protect a company in the case of future liability claims. Air monitoring, using battery-powered vacuum pumps with filter cassettes, will provide factual information on air quality. Air samples should be collected by a qualified person to ensure correct sampling procedures and reliable results.
One potential problem may be the existing ventilation system not operating at full potential. Check for broken or leaking ducts, clogged airways, and full dust collection bins that may be restricting the air flow. The system may be outdated or undersized due to changes or expansion of production facilities. If this is the case, certain modifications may be made to bring it to current standards (American National Standard Institute standard Z33.1-1961). The ventilation system may be adequate, but if improperly fitted, it may not be intercepting all dust being produced.
Something as straightforward as equipment layout may also be a source of wood-dust problems. The rotating blades of a planer may throw dust and wood chips back toward the operator. In some situations, the back edge of the saw blade of a circular saw can create dust that may be directed by the guard and projected towards the operator. A crosscut saw usually creates a jet of dust that may be directed towards another workstation, creating higher levels of dust at that station. Evaluate the dust generation patterns of existing and planned equipment and arrange equipment in the shop to reduce any problems.
Employers should take note of employees' complaints of shortness of breath and whether employees have been missing work because of dust exposure. Employees' past work histories may reveal exposures to wood dust and other air contaminants in previous jobs. Past and present health records can be compared to determine any changes in the respiratory health of workers that might suggest the need for improvements.
Chan-Yeung, M. 1994. Mechanism of occupational asthma due to western red cedar (Thuja plicata). American Journal of Industrial Medicine 25:13-8.
Department of Labor. Jan. 1989. Federal Register, pp. 2528-2533. January.
Mazurkiewicz, Michael, M. S. and J. L. Festa, 1989. "Study Evaluates Wood Dust Exposure in U.S. Plants." Wood and Wood Products. p. 158. July.
Meola, A. March 1985. "Toxic effects of Wood Dust Exposure." Professional Safety.
Milham, S. 1974. "Mortality Experience of the AFL-CIO United Brotherhood of Carpenters and Joiners of America, 1969-1970: Division of Field Studies and Clinical Investigations." NIOSH Publication No. 74-129. NIOSH, Salt Lake City, Utah.
Milham, S. and Hesser, J. E. 1967. "Hodgkin's Disease in Woodworkers." Lancet, Vol 2: 136-137.
"National Forest Products Assn. and Inter-Industry Wood Dust Coordinating Committee Seminar Review." 1989. Wood and Wood Products, p. 166. July.
Spiers, P. S. 1969. "Hodgkin's Disease in Workers in the Wood Industry." Public Health Reports, 84(5): 385-388.
Reviewed by Dr. Curt Hassler, WVU; Dr. Wayne Maines, WVU; Dr. Thomas G. Carpenter, OSU; and Dr. Randall K. Wood, OSU.
Originally funded in whole or in part from Grant Number U05/CCU506070-02,"Cooperative Agreement Program for Agricultural Health Promotion Systems," National Institute for Occupational Safety and Health. Timothy W. Butcher, OSHA Program Coordinator; Thomas L. Bean, Safety Leader.
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